Polylactic acid (PLA) is a thermoplastic polyester material. PLA is biodegradable in human body and can be naturally excreted from the body after being decomposing into CO2 and H2O. In the medical field, PLA has been used to manufacture medical material such as bone screws, bone plates, vascular stents, surgical sutures, surgical guides, drug delivery materials, artificial skin, and wound dressings for medical purposes. These bone screws can be directly implanted into human body to treat bone diseases, where it can be naturally degraded in human body, thereby sparing the necessity of second surgery for removing the bone screws.
Micron-sized ferroferric oxide particles are material with ferrimagnetic properties. While ferroferric oxide particles become smaller than 50 nanometer, they have superparamagnetic properties. This unique property enables nano-sized ferroferric oxide to be well applicable in the biomedical field and the biological engineering field as well. The applicable fields include magnetic bead separation technology, hyperthermia, drug delivery systems, and magnetic resonance imaging (MRI) contrast agents. In addition, a nano-sized ferroferric oxide particles exhibit osteogenic property.
The use of PLA in the manufacture of bone screws is conventional technology, wherein the manufacturing method is achieved by heating PLA to 180 to 198 degrees and then injecting the PLA into a bone screw mold to make the PLA form a bone screw. A Bone screw made of pure PLA is not radiographable under X-ray exposure due to its low mass density and low electron density, and thus it is hard to perform surgical positioning and the evaluation of postoperative degradation when bone screws made of pure PLA are used in surgery. In order to overcome the drawback, some additives such as barium sulfate and bismuth bromide are added into high molecular polymer to overcome the low radiographable property. However, because of the poor biocompatibility that these additives possess, they can cause harmful side effects in human body since the added additives will be released after the high molecular polymer of biodegrade.
Examples of binding high molecular weight polylactic acid to ferroferric oxide nanoparticles can be found in a Chinese utility model patent with publication number CN 103360607A and CN 103360607B, which disclose a method for the preparation of magnetically-responsive star-shaped block copolymer nanomicelle, comprising: using an magnetic iron oxide (Fe3O4-OH) nanoparticles modified by γ-glycidoxypropyltrimethoxysilane (KH-560) as a nucleation initiator, triggering lactide (LA) to perform active ring opening polymerization by using hydroxyl groups on the surfaces of magnetic nanoparticles and then obtaining a magnetic star-shaped polymer as a nucleus, and esterifing the star-shaped polymer with carboxylated methoxy-polyethylene glycol(CMPEG) to synthesize a magnetically-responsive star-shaped block copolymer nanomicelle. The purpose thereof is to prepare a magnetic high molecular weight polymer material. In addition, another example of binding high molecular weight polylactic acid to iron oxide nanoparticles can be found in Chinese utility model patent with publication number CN 103745793 A, which discloses a method for the preparation of a targeted superparamagnetic liposome-polylactic acid (PLA) nanoparticles and method for the preparation thereof, comprising: forming PLA-phospholipid-PEG nanoparticle by coating polylactide on Fe3O4 as a PLA core, coating a single layer of phospholipid on the surface of the PLA core, and alternately adding distearoyl phosphatidyl ethanolamine-polyethylene glycol-carboxylic acid in the single layer of phospholipid. The purpose thereof is to develop a magnetically adsorptive material for separating tumor cells. However, these preparation methods cannot be applied to the preparation of a bioabsorbable PLA bone screw which is radiographable under X-ray exposure.
Therefore, it is the object of the present invention to provide a bone screw which is radiographable under X-ray exposure so as to overcome the problem of the conventional bone screws which is radiopaque. The present invention is also with advantage of being bioabsorbable such that the bone screw, while being radiographable, can still be degraded in human body in such a manner that it can spare the necessity of second surgery for removing the bone screws.